Designing or adapting an electric motor with a rotating shaft for deep ocean use is problematic. The difficulty lies in sealing the motor against the extreme high pressure of the ocean to prevent leakage of sea water into the electric motor.
One currently available way of designing or adapting an electric motor for deep ocean use is to continuously monitor the ocean pressure the motor is operating in and to equalize the pressure within the motor itself to the ocean pressure. This is generally accomplished by filling the motor with oil and using a bellows diaphragm to transmit the pressure. A commercial shaft seal is typically used to prevent the sea water from entering the motor because the differential pressure between the ocean water and the electric motor is zero. However, due to the presence of oil within the motor, commutation is carried out at great cost and complication and the complex electronics needed to accomplish this are usually sealed in a separate black box. Currently, a 0.75 horse-power electric motor utilizing this method of commutation costs between $15,000 and $18,000.
Another method of designing or adapting an electric motor for deep ocean use is to initially fill the motor with oil at deep sea pressure and use a regular commutator of carbon brushes and copper segments running in the oil. These motors cost significantly less then the complex motors described above, unfortunately however, arcing and sparking of the commutator causes quick deterioration of the oil and commutator parts. Thus, this method has a short useful life.